Color filters are produced by forming a color pattern by photolithography or the like using a curable color composition containing a pigment dispersion composition in which an organic pigment or an inorganic pigment is dispersed, a multifunctional monomer, a photopolymerization initiator, an alkali-soluble resin, and other components.
In recent years, there has been a trend for the liquid crystal display (LCD) applications of color filters to be expanded to television (TV) applications as well as monitor applications. With this trend toward expanded applications, high levels of color properties are required with respect to chromaticity, contrast and the like. Further, in image sensor (solid-state imaging device) applications, color filters are similarly required to have high levels of color properties, such as a reduction in color unevenness and improvement of chromatic resolving power.
With regard to the above requirements, it is required that the pigment contained in a curable color composition be dispersed in a finer state (favorable dispersibility) and be dispersed in a stable state (favorable dispersion stability). In particular, making the primary particle diameter of the pigment very small is effective.
An example of a method for making the primary particle diameter of the pigment very small is a method of mechanically kneading the pigment, together with a synthetic resin that is solid and water-insoluble at room temperature, a water-soluble inorganic salt such as common salt and a water-soluble organic solvent that dissolves at least a part of the synthetic resin, using a kneader or the like (kneading of a mixture containing a pigment, a water-soluble inorganic salt and a water-soluble organic solvent is hereinafter referred to as “salt milling”), and thereafter removing the inorganic salt and the organic solvent by washing with water (see, for example, Patent Document 1). In this method, since pulverization of primary particles of the pigment and crystal growth occur in parallel during the process, a pigment having a narrow particle size distribution and having a surface area that is small even for its small average particle diameter is ultimately obtained; therefore, this method is a method of producing a pigment that is useful for applications, such as color filters, in which a pigment having a very small particle size has to be dispersed at high concentrations.
However, when the primary particles of the pigment are made finer, the particles are more likely to aggregate, and are more likely to form an aggregate (secondary aggregate) when in a state of slurry or when dried. Further, as the primary particles of the pigment are made finer, strong secondary aggregation is more likely to occur. Therefore, it is generally very difficult to re-disperse the finer pigment as primary particles.
When the dispersibility of the pigment is insufficient and aggregates and the like are present, a color film formed using a curable color composition containing the pigment has poor uniformity, and fringes (indents at an edge portion) and surface irregularity occur at the curable color film, as a result of which conditions such as generation of color density unevenness, a reduction in dimensional accuracy, and significant deterioration of contrast may arise in a produced color filter.
When the dispersion stability of the pigment is insufficient, there is a problem in that constituent components of the curable color composition, including the pigment, aggregate and cause an increase in viscosity over time, as a result of which the pot life becomes extremely short. When producing a color filter using such a curable color composition, the uniformity of the film thickness in the coating process is reduced, which is likely to cause problems such as sensitivity variations in the exposure process, alkali-solubility variations in the development process and non-uniformity. In particular, if slit coating (or die coating) is performed when forming a color film formed from a curable color composition on a glass substrate in order to produce a color filter with a large surface area, there has been a problem in that defects such as streaks easily occur on the coating surface, and the properties of the coating surface decline, which leads to pixel defects in the color filter.
In order to improve the dispersion stability of a pigment obtained by applying the salt milling method, a method in which a rosin or rosin derivative, or a synthetic polymer, is added at the time of salt milling has been proposed (see, for example, Patent Document 2). A method in which the dispersion stability is improved by additionally using a specific pigment derivative or a dispersant containing a partial skeleton of an organic pigment in the dispersing process has been proposed as another measure to improve the dispersion stability (see, for example, Patent Documents 3 and 4).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 7-13016    Patent Document 2: JP-A No. 8-179111    Patent Document 3: JP-A No. 2007-186681    Patent Document 4: JP-A No. 2003-26950